I buy your bitcoin at $4.000! Bitcoins are now sold for around $8.500 and I think the question you should ask is, who want to sell their bitcoins for just $ 8.500?
people who bought Bitcoins at $4000 will sell at $8500 for the easy money. Just buy and sell.
Yes that's a good money coming from be part of the cryptocurrency community. Although, you must be careful of when to buy and when to sell, hence such individuals or investors are liable to looses. There's still more pretty chance that the price may drop further and when this happens ensure to buy and hold for another bull rise.